This invention relates to an optical scanner for reading a bar code or the like making use of a hologram and a laser beam, and more particularly to an optical scanner for a POS (point-of-sales) terminal.
One of various types of optical scanners which scan a laser beam in accordance with a desired pattern is a hologram scanner which employs a hologram disk as a scanning means. Such employment of a hologram disk as a scanning means enables formation of a complicated scanning pattern with a simplified optical system and realization of bar code readers having a deep reading depth.
A POS bar code reader (POS scanner), which is one of various types of optical scanners, moves across a bar code applied to a commodity above a reading window to read the bar code information with a laser beam and is constituted from a laser beam generating light source, a laser beam shaping optical system, a scanning optical system, a signal light detecting optical system, a waveform shaping circuit and a bar code symbol demodulating circuit. A laser beam emitted from a He--Ne laser is shaped into a beam of a suitable diameter by the beam shaping optical system and then scanned to form a universally readable scanning pattern by the scanning optical system, and a bar code is irradiated with the scanning pattern. Scattered light reflected from the bar code is condensed by the signal light detecting optical system, in which signal light is converted into an electric signal by a photo-detector. The electric signal is shaped by the signal waveform shaping circuit and then converted by the bar code symbol demodulating circuit into numerical values, which are then sent to a POS terminal.
A prior art optical scanner is disclosed in U.S. Pat. No. 4,848,862 wherein a rotary polygon mirror is employed as a laser beam scanning means and strip holograms of the transmission type are employed for a reading window. An outline of the prior art optical scanner will be first described with reference to FIGS. 1, 2A and 2B. Referring first to FIG. 1, a reading window generally denoted at 10 is composed of three transparent substrates 11, 12 and 13 having strip holograms 11a, 12a and 13a, respectively, of the transmission type formed in different directions from each other and adhered in layers to each other such that the strip holograms 11a, 12a and 13a of the transmission type thereof may extend in an intersecting relationship to each other. As shown at a lower portion of FIG. 1, located below the reading window 10 are a scanning pattern generating mirror means 14 consisting of three side mirrors 15, 16 and 17, a concave mirror 18 having a through-hole 18a formed therein and having a curved reflecting face on an inner surface thereof, a bottom mirror 19 disposed in parallel to the reading window 10, a photo-detector 20, a mirror 21, and a polygon mirror 23 having five reflecting faces and connected to be driven to rotate by a motor 22. Such optical parts as listed above are mounted in a predetermined positional relationship together with a He--Ne laser tube 24, a beam shaper 25 and a reflecting mirror 26 on a base not shown to generally constitute the optical scanner.
Operation of the optical scanner will be described subsequently with reference to FIGS. 2A and 2B. A laser beam emitted from the laser tube 24 is first shaped in beam diameter thereof by the beam shaper 25 and then reflected toward the concave mirror 18 by the reflecting mirror 26. As shown in FIG. 2A, the laser beam 28a reflected by the reflecting mirror 26 passes through the through-hole 18a of the concave mirror 18 and is then reflected by a back mirror 27, whereafter it passes through the through-hole 18a again and is introduced to the polygon mirror 23. The laser beam 28b is then scanned within a predetermined range in accordance with an inclination of reflecting faces and rotation of the polygon mirror 23 so that it makes scanning laser beams 28c and 28d which successively scan the three side mirrors 15, 16 and 17. The scanning laser beams 28c and 28d are projected toward the reading window 10 by way of the side mirror 15, 16 or 17 and the bottom mirror 19 to successively scan the three strip holograms 11a, 12a and 13a of the transmission type which are different in direction from each other. The laser beams 28e and 28f diffracted by any of the transmission type strip holograms 11a, 12a and 13a are projected as scanning lines of predetermined directions, and a desired scanning pattern is formed by such laser beams 28e and 28f.
On the other hand, as shown in FIG. 2B, signal light from a bar code affixed to a commodity is diffracted by the reading window 10 and introduced to the bottom mirror 19 and then reflected successively by the bottom mirror 19, side mirror 16 and polygon mirror 23 so that it is introduced to the concave mirror 18. The scattered light signal is condensed and reflected by the concave mirror 18 and then introduced by way of the mirror 21 into and detected by the photo-detector 20.
With the optical scanner having such a construction as described above, since a laser beam is projected from the reading window 10 such that a plurality of scanning lines having different directions may intersect each other in every plane above the reading window 10, no distance is required between the reading window and a bar code to be read. Consequently, reduction in thickness of the apparatus can be attained.
However, the optical scanner disclosed in U.S. Pat. No. 4,848,862 necessitates three horizontally divided side mirrors incorporated below the reading window in order to generate three scanning lines of different directions above the reading window. Since the two opposite side ones of the three side mirrors are disposed such that they extend outwardly of the reading window, the outer profile of the optical scanner apparatus must be made greater than the size of the reading window. Accordingly, the optical scanner is disadvantageous is that the entire apparatus cannot be made compact sufficiently. Besides, since the optical scanner has a two-story structure wherein the centrally located side mirror is disposed on the concave mirror, it is disadvantageous in that the entire apparatus cannot be reduced in thickness sufficiently.